An organic light-emitting diode (OLED) is a light-emitting diode employing an organic light-emitting layer as an active layer. OLED display devices have high luminescent efficiency and long operating lifespans. In comparison with liquid-crystal displays, due to the characteristics of spontaneous emission, a device employing an organic light-emitting diode is free of a back-light source.
Generally, an organic electroluminescent device is composed of a light-light-emitting layer sandwiched between a pair of electrodes. When an electric field is applied to the electrodes, the cathode injects electrons into the light-emitting layer and the anode injects holes into the light-emitting layer. When the electrons recombine with the holes in the light-emitting layer, excitons are formed. Recombination of the electron and hole results in light emission.
Depending on the spin states of the hole and electron, the exciton can have either a triplet or singlet spin state. Luminescence from a singlet exciton results in fluorescence whereas luminescence from a triplet exciton results in phosphorescence. The luminescent efficiency of phosphorescence is three times that of fluorescence.
Therefore, it is crucial to develop highly efficient phosphorescent material in order to increase the luminescent efficiency of an OLED.